Molecular and cellular disorders AHS

Nucleotide (3)

Polynucleotide (4)

DNA replication (4)

DNA compaction (4)

• monomer (sugar, phosphate, nitrogenous base)

• complementary base pairing joins stands to form DNA (adenine-thymine, guanine-cytosine, hydrogen bond between bases)

• Cell cycle synthesis, parental stand as template, new strands synthesised, daughter molecule one old one new stand (semi-conservative replication)

• DNA, around histones, folding to chromatin, folds to chromosome

Chromosomes (5)

Dog, Cat, Cow

• Compact, protects DNA from damage, organisation and structure, efficient transmission during cell division, each pair has same banding latter and centromere position

• Dog 39 pairs (78), Cat 19 pairs (38), Cow 30 pairs (60)

Karyotype

Cytogenetics

Aneuploidy/ Trisomy/ Monosomy

• All chromosomes arranged in pairs based on length from longest (consistent comparisons)

• Detection and interpretation of chromosomal abnormalities, changes to chromosome number or structure, associates with physical or mental abnormalities

• Dif number of one chromosome/ one additional copy of one chromosome/ one copy of one chromosome

Mitosis

Meiosis

Genome

Genetic mutations

Phenotype

Genotype

• Complete set of genetic material found in an organism, includes nuclear and plastid DNA, contains coding and non-coding DNA

• Are permanent and heritable changes or disruptions in the base sequence of the genome that alter the expression/structure/function of the coded protein

• Clinical manifestation of specific gene

• Specific combination of alleles an individual has

Aspects at which faulty genes cause disease (central dogma) (5)

• DNA replication

• Transcription (exons)

• Translation

• Additional elements affecting successful expression of gene (prior to gene in DNA)

• Multiple transcription factors

Single gene disorders:

Factor VII deficiency (FVIID)

• clotting disorder, results in mild breeding episodes and bruising

• Lack of proconvertin (clotting factor 7)

• Gene mutation (GGA to GAA), glutamic acid (not glycine) in exon 5, polypeptide altered, change in EGF2 domain structure, reduced function (interacts with tissue factors in coagulation cascade)

Mendelian inheritance:

The law of segregation

Law of independent assortment

Genetic linkage

Sex chromosomes

• Organisms inherit two alleles for each characteristic from each parent, two alleles for each characteristic segregate at gamete production

• Inheritance pattern of one trait will not affect the inertiance pattern of another

• Tendency of DNA sequences that are close together on a chromosome to be inherited together during meiosis

• Gene located on differential regions of sex chromosome (most on X), pseudoautosomal region (allows correct lining up) and differential regions (not found on other sex chromosome)

Codominance

Incomplete dominance

Epistasis

Lethal alleles

Penetrant

• Both dominant genes expressed, equally expressed

• Neither allele is dominant, blended expression

• Expression of one gene is affected by expression of one or more independently inherited genes

• Essential to life, one copy of recessive lethal allele is survival with defects, two recessive or dominant lethal alleles is death

• Proportion of individuals with genetic variant/mutation who exhibits signs and symptoms of genetic disorder

Chromosomal mutation (4)

DNA replication mutation (3)

Spontaneous/ Induced mutation

Point mutations (substitution 3, insertion/ deletion)

• Deletion, duplication, inversion, translocation

• Mistakes in DNA replication base pairing (proofreading and post-replication mismatch repair to increase accuracy)

• Occur in natural environment without addition of mutagens at low rate/ created by addition of mutagens

• Alters sequence of bases within genes

  -One base changed to another

  -Missense mutation (mutated codon for incorrect amino acid)

  -Nonsense mutation (mutated codon for stop codon)

  -Silent mutation (amino acid sequence not affected)

  -Adding or removing once base, inappropriate number of bases in a DNA sequence, affects reading frame onwards (frame shift mutation, function affected)

Canine haemophilia B

Bovine leukocytes adhesion deficiency (BLAH)

Hyperkalemic periodic paralysis (HYPP) horses

Porcine stress syndrome/ malignant hyperthermia

X-linked tremors/ shaking puppy syndrome

• Sex linked recessive missense mutation, substitution of A for G at nucleotide 1477, reduced coagulation

• Autosomal recessive missense congenital disease, substitution of ITGB2 gene on chromosome 1, leukocytes can’t emigrate to sites of injury from circulation

• Autosomal dominant congenial disease, substitution in SCN4A gene, episodic electrical inexcitability and paralysis of skeletal muscle as sodium channels effected

• Missense mutation in ryanodine receptor gene

• Missense mutation in preteolipid protein gene, coarse tremors of the head and body, lack of myelin in CNS

Mutagens:

Physical (3)

Chemical (3)

Biological (3)

Effects (6)

• Radiation, UV light, x rays

• Base analog (similar to nucleotide), Base altering agents (induce methylation, alkylation or deamination of DNA bases), Intercalating agents (similar to base pair heterocyclic ring)

• Transposons and insertion sequences, viruses, bacteria

• Direct DNA or nucleotide damage, induce cross-linking (bases which shouldn’t link), break chromosomes, break DNA strand (single or double), base deletion, nucleotide dimers (same bases joined if closed together)

Types of mutations:

Somatic (3)

Germ-line (3)

• In non-reproductive cells, may not be manifested, can result in cancer if altered cell division patterns

• In gametes or reproductive cells that produce gametes, heritable, transfer to next progeny in all cells

Types of alleles (4)

• Homozygous dominant AA

• Heterozygous A (carriers)

• Homozygous recessive aa

• Hemizygous (males for sex linked chromosomal disease as they can only have one X copy)

Modes of inheritance:

Autosomal recessive

Autosomal dominant

X-linked recessive

X-linked dominant

• Two copies for presentation, asymptomatic carrier parents, 25%/ 1 in 4 affected offspring probability, can skip generations, Myoclonic epilepsy in Rhodesian ridgebacks (defective DIRAS1 gene, longer AA sequence bc of stop loss)

• One copy needed for presentation, often lethal if two copies present, no generation skipping, defective product usually structural protein, Polycystic kidney disease in Persian cats (point mutation in PKD1 gene on chromosome 3)

• Consider which parent has abnormal gene, females only express if abnormal from both parents, female carriers if one, males affected if recessive allele present from mother, Haemophilia A and B (coagulation cascade factors 9 and 8)

• Consider which parent has abnormal gene, females affected unless homozygous recessive, males affected if dominant allele present

Genetic mosaicism

X inactivation

Pedigree

Mode of inheritance using pedigree

• Mutation during embryogenesis, amount and location of mosaicism determined by timing and lineage of affected somatic cells

• Random inactivation of X chromosome alleles post division, leaves two different alleles active, Tortoiseshell coat in female cats

• Chart of genetic history of related individuals over several generations, used to predict likelihood of disease progeny, determine mode of inheritance and probability of affected offspring for given gross

• X linked disorder (mostly males effected), Autosomal disorder (even between males and females), Dominant (one parent must have disorder), Recessive (neither parent has disorder)

Inborn errors of metabolism

• Result of single gene disorders

• Cellular metabolism has many enzymatic pathways (cascade or inhibition) with many steps, each requires enzyme or protein with specific function

• Non-functional proteins stops pathway, results in product accumulation, may have toxic effects

Citrullinemia (single gene disorder):

Clinical signs

Cycle effected

Mutation

Effects

• Clinical sings 1-4 days (blindness, depression, convulsions, head pressing)

• Urea cycle (liver converts ammonia to urea for excretion vis kidneys), ASS enzyme converts citrulline to argininosuccinate

• Substitution of T for C on first position of 86th triplet, nonsense mutation (stop codon) terminated at 85th AA, ASS not functional

• Autosomal recessive disease, carriers have 50% ASS activity compared to normal, affected have 0% ASS activity, buildup of toxic ammonia in blood

Ehlers-Danlos syndrome (single gene disorder):

Clinical signs

Structure

Mutation

• Born with stretchy or very fragile skin

• Collagen fibres form from triple-helix pro-collagen, requires enzymatic modification to form mature functioning collagen, contains a-1 chains and a-2 chains, coded for by separate genes, pro-collagen forms collagen by removal of terminal AA

• Mutation can occur in a-1 collagen chain gene, a-2 collagen chain gene, PCP-C-1 endopeptidase gene, PCP-N-1 endopeptidase gene

Genetic heterogeneity:

What (2)

Example

• Specific set of clinical signs arises from more than one mutation

• More than one mutation in the same gene that causes the same clinical effects

• Ehlers-Danilo’s syndrome (dominant if mutation in chain genes, recessive if mutation in endopeptidase genes)

Progressive retinal atrophy PRA: Progressive Rod-Cone Degeneration PRCD(single gene disorder):

Clinical signs

Structural change

Mutation

Diagnosis

• Late onset age varied between breeds, night blindness to total blindness

• PRA causes retinal cells to degenerate and die, initial loss of rods then cones

• Autosomal recessive, mutation in PRCD gene, G to A at chromosome 9

• Genetic tests, ophthalmic evaluation after first clinical signs, electroretinography (ERG) very sensitive indicator of PRA

Neoplasia

Carcinogen

Angiogenesis

Metastasis (4 types)

Heterogeneity

• Uncontrolled cell growth/ cell cycle leading to neoplasm/tumour, preventative strategies avoided, occurs in any tissue type/ body system in body system

• Substance capable of causing cancer or promoting cancer in living tissues, increase risk, causes DNA damage causing mutations leading to cancer

• Secretion of growth factor to stimulate blood vessel growth to supply tumour cells, anti-angiogenic signalling overwhelmed by pro-angiogenic signalling

• Malignant cells transfer to another part of body, acquisition of invasive phenotype, in circulation (to organs with large blood supply), in lymphatic drainage system (seed in lymph nodes), extension (to neighbouring organ via direct contact), transplantation (exfoliation of tumour cells into a cavity or another site)

• Evolution of tumour to form different clonal stem cell lines, additional mutations (resulting in dif genotypes and phenotypes of cancer cells), selection pressure (cancer drug introduction results in resistance due to new genotype)

Control of cell cycle:

Cell response

Checkpoints

Inhibitory proteins

Stimulating proteins

Other protein

• Divide/ stop dividing , differentiate, die as response to molecular signals

• G1 cell size (rest or divide), S (DNA ok), G2 size and organelles (fully equipped), M (lined up), progression regulated

• P53 tumour suppressor/ tumour suppressor gene, stops progression, allows repair time, triggers apoptosis

• CDK-cyclins/ protocol-oncogene, activate or inhibit protein via phosphorylation, activate cell cycle stimulating proteins

• DNA repair proteins, various genes

Tumour biology:

Mutation

Hyperplasia

Dysplasia

Anaplasia/ Carcinoma in situ

Metastasis/ Invasive carcinoma

• Genetically altered

• Rapid/ uncontrolled cell division

• Abnormal change

• Lose function, spread over wide area

• Invades normal tissue, enter blood and lymph, metastases form in other sites

Benign tumour/ neoplasms (what, 2 examples)

Malignant tumour/ neoplasms (what, 2 examples of each)

Mast cell tumours

Leukemias

Myelomas

• Localised, slow growth/ few mitosis figures, circumscribed/ encapsulated, resemble original tissue, -oma

  -Lipoma (adipose cells), Fibroma (fibrous connective tissue)

• Invade and metastasis to other sites, rapid growth/many mitosis figures, non encapsulated, don’t resemble original tissue (anaplastic), -carcinoma (epithelial origin) -sarcoma (mesodermal origin)

  -Transitional cell carcinoma (urinary tract), Adenocarcinoma (glandular tissue/ mammary glands), Osteosarcoma (osteoblasts), Lymphosarcoma (lymphoid tissues)

• Mast cells, usually affects skin, contain histamine so highly reactive when irritated by handling

• Hematopoetic cells in peripheral blood or bone marrow, cell lineage identification important step in classification of leukemia

• Plasm cells/ terminally differentiated B lymphocytes, results in diffuse disease

Cancer hallmarks:

Original (6)

Emerging (2)

Enabling (2)

• Sustained proliferation signalling, evading growth suppressors, enabling replication immortality, activating invasion and metastasis, inducing angiogenesis, resisting cell death

• Avoiding immune destruction, deregulating cellular energetics

• Tumour-promoting inflammation, genome instability and mutation

Polygenic traits:

What

Behaviour

Coat colour

• Phenotype results from joint contribution of multiple independently acting or interacting genes

• 131 SNPs associated with breed differences in behaviour (genes expressed in brain with neurological function and developmental processes), owner aggression, stranger fear, trainability

• Dif allele combinations from dif loci, dif promoter alleles drive dif colour patterns/ have mutations, action of one gene depends on action of another gene (ASIP gene causes pigment change from black/brown to yellow/white)

Multifactorial traits/ complex inherited disorders:

What

Liability (bell curve)

Hip dysplasia

Equine metabolic syndrome

• Caused by multiple genes and influenced by environment, (Cancer)

• Combined effect of all factors (environmental and genetic) that render animal more or less likely to develop that disorder

• Hip dysplasia gene, food level, dietary electrolytes, exercise level, subjectivity of radiographs

• Insulin dysregulation genes (dominant, multiple, reduced penetrance, possible epigenetic influences), obesity, dietary sugar, exercise, concurrent illness

Heritability:

What

Aspects (4)

Examples (3)

Non-genetic alleviation

• Proportion of total variation in a trait that can be attributed to variation in genetic factors

• Relative importance of genetic and environmental factors, contribution of each, heritability for liability usually greater than 0 (liability based on genes), selection for decreased liability can reduce incidence

• Metritis in cows 5, Hip dysplasia in dogs 35, tibial dyschondroplasia in chickens 45

• Inherited disorders can be alleviated by non-genetic means even when heritability of liability is high (restricted feeding, exercise during growing phase for hip dysplasia)